The present invention relates in general to analog-to-digital convertors and in particular to a new and useful voltage-to-pulse convertor which converts an analog signal into a pulse having a duration which is proportional to the analog signal.
Signal conversion of an analog to digital signal for digital processing in low power applications, such as pressure transducers is normally accomplished by a voltage to frequency convertor (V/F). Commercially available A/D convertors are not utilized due to their excessive power consumption. There are not commerically available integrated circuits for analog to digital conversion that are of sufficient low power, that is less than 12 mW, to run from a 4-20 mA current loop.
While V/F convertors can be utilized, they are relatively slow in response time and low in accuracy. The slow conversion of the V/F convertor results from the need for a counting interval that is at least as long as the period for the lowest frequency signal. The response time is a fixed value regardless of the frequency of the signal. This means that a microprocessor used in conjunction with the V/F converter will be tied up during this conversion time. This limits the overall response of the pressure transducer.
With regard to accuracy, a moderate-precision V/F circuit has an accuracy range of about 0.4% to 0.1%. To obtain a high-precision V/F circuit with an accuracy range of 0.03% to 0.01% requires additional components which result in greater power consumption and higher cost.
A convertor which operates at low power consumption and with high accuracy would be particularly useful for a two-wire 4-20 mA current loop.
Two-wire analog transmission systems are well known. Such systems include a transmitter which is connected to a power supply by two wires which form a current loop. The transmitter includes, as at least one of its features, a transducer which senses a condition such as pressure or temperature. This condition is known as a process variable (PV).
A power supply is connected to the two wires to close the current loop. It is also conventional to provide a resistor in the current loop. The transmitter amplifies the signal from its transducer and this amplified signal is used to draw a certain current from the power supply which is proportional or otherwise related to the process variable. It is conventional to draw from a minimum of 4 (mA) to a maximum of 20 mA. The current between 4 and 20 mA passes through the resistor to produce a voltage drop across the resistor. This voltage drop can be measured to give a value for the process variable.
It is noted that the 4 mA minimum current is required to energize the circuitry of the transmitter. Any excess current above this 4 mA level is taken as a value which can be used to determine the process variable.
It is known that such 4-20 mA two-wire systems have an accuracy which is limited to around 0.1% at best. These systems are also essentially unidirectional with the transmitter being essentially uncontrolled and transmitting continuously.